The present invention relates to a method and apparatus for measuring stress in semiconductor wafers, and more particularly but not exclusively to measuring stress in an integrated operation or apparatus together with thickness, in semiconductor wafers.
The semiconductor chip manufacturing process generally involves forming a silicon wafer and then carrying out a sequence of operations that essentially involves addition and selective removal of layers to build the functionality of the chip.
Layer thickness and layer stress are two measurements that are carried out on the silicon wafers at various stages of the manufacturing process. In particular, in the semiconductor manufacturing process, there are several steps in which dielectric layers are deposited on the silicon wafers as part of the above-mentioned sequence.
Following deposition, several properties of the layer may then be tested to verify both the quality of the layer itself and the quality of the deposition process. In general, thickness and stress are measured separately and in different ways. The two measurements increase the time of the process, thereby reducing throughput, and also increase the floor space required to accommodate the process.
Concerning thickness measurement, PCT Patent Application No. WO 0012958 describes a measurement system, known as TMS, which uses light beams reflected from points within layers of a wafer surface to make measurements of the thicknesses of transparent layers and in particular photoresist layers. The measurement is transformed typically into the frequency domain from which it is possible to determine the photoresist layer thickness very accurately. The measurement is rapid and simple.
Moving on to stress measurement, when a thin film is deposited onto a substrate such as a semiconductor wafer, mechanical stress is built up on both the film and the substrate. The stress may cause the wafer to bow as well as induce cracks, voids, hillock formation and film lifting, leading to reduced yield and lowered reliability.
A known method of measuring stress levels that develop when a thin film is deposited onto a substrate such as a semiconductor wafer, involves measuring the bow of the wafer as a whole prior to deposition of the layer, storing the result, and then measuring it again following deposition of the layer.
Determining the bow may be carried out using a laser, or more generally a monochromatic light beam, reflected from the wafer for measurement of net radius of curvature R. A stress level may be determined by the known formula:       Stress    =                  EDs        2                    6        ⁢                  (                      1            -            v                    ⁢                      xe2x80x83                    )                ⁢        DfR              ,
Where
E=Young""s modulus of the substrate,
v=Poisson""s ratio for the substrate,
Ds=thickness of the substrate,
R=net radius of curvature,
Df=thickness of film.
Incompatibility of the light sources, and also the need to carry out the first part of the stress measurement prior to deposition of the layer precludes integrating the thickness and stress measurements. Generally the two measurements are carried out separately in time and using separate measuring devices.
According to a first aspect of the present invention there is thus provided apparatus for measuring properties of a wafer, the apparatus comprising:
at least one monochromatic light source, for producing monochromatic light for directing at the wafer,
at least a first and a second beam director, each optically associated with the monochromatic light source for directing the monochromatic light beams as respective substantially parallel light beams towards the wafer, each to strike at respective places on a planar surface of the wafer, the first beam director being arranged to direct a respective beam substantially centrally onto the planar surface and the second beam director being arranged to direct a respective beam substantially outwardly of the center, and
an optical processor arranged to receive reflections of each beam and to process each reflection, therefrom to obtain an optical difference between the reflections, the optical difference being indicative of bow in the wafer,
The apparatus preferably comprises a measurement mode switch for switching between the bow measurement and measurement of another property of the wafer.
The apparatus preferably comprises an output operatively associated with the optical processor for outputting an indication of stress in the wafer, the indication being based on the bow.
The beam directors preferably comprise optical heads for receiving light from the monochromatic light source and directing the light as beams to the wafer.
The optical heads are preferably operable to receive the reflections from the wafer and to direct the reflections to the optical processor.
Preferably, the first beam director is arranged to direct light to strike the planar surface substantially perpendicularly.
Preferably, at least the second beam director is arranged to direct light to strike the planar surface substantially obliquely.
Preferably, the optical processor comprises an interferometer.
Preferably, the interferometer is operable to produce an interference pattern between the beams, the optical difference being a path difference between the beams.
Preferably, the interferometer is a Michelson interferometer.
Preferably, the optical processor comprises a beam reflector and a plurality of waveguides, the beam reflector being arranged to deflect reflections from the planar surface to the waveguides such that intensity of the deflection is differentially distributed between the waveguides as a function of an angle of the reflection.
Preferably, the optical processor further comprises an intensity measurer for measuring light intensity in each of the waveguides, the optical difference being a difference in distribution of intensity over the waveguides.
Preferably, the optical beam reflector is an off axis parabolic reflector.
Preferably, the monochromatic light source comprises a laser.
The apparatus preferably comprises a stress determination unit operable to determine a stress level from the bow using the relationship       stress    =                  ED        2                    6        ⁢                  (                      1            -            v                    )                ⁢        tR              ,
wherein
E=Young""s modulus of the wafer,
v=Poisson""s ratio for the wafer,
Ds=thickness of the wafer,
R=net radius of curvature, derivable from the bow, and
Df=thickness of a film deposited on the wafer.
Preferably, another property is thickness, the apparatus further comprising a white light source, each the beam director comprising a first optical switch to select between the white light source and the monochromatic source.
The apparatus preferably comprises a second optical processor and at least one second optical switch, the second optical switch being arranged to select between the first optical processor and the second optical processor to direct the reflections to the selected optical processor.
Preferably, the second optical processor comprises a spectrometer.
Preferably, the first and second optical switches are controllable together to select the monochromatic light source with the first optical processor and the white light source with the second optical processor.
The apparatus preferably comprises a Fourier transform device connected to the spectrometer for transforming an output of the spectrometer into the frequency domain, thereby to obtain information of layer thickness of at least one deposited layer on the wafer.
According to a second aspect of the present invention there is provided an integrated measurement apparatus for measuring layer thickness and bow in a wafer, the apparatus comprising:
a monochromatic light source,
a white light source,
a first switch for switching between the white light source and the monochromatic light source,
a plurality of beam directors for directing light from the switched light source onto a semiconductor wafer,
a first optical processor for spectral processing of reflected light from the wafer,
a second optical processor for processing of reflected light to determine an extent of bow in the wafer, and
a second optical switch to switch reflected light from the wafer between the first optical processor and the second optical processor.
Preferably, the first optical processor is a spectrometer and the second optical processor is an interferometer.
Preferably, the first optical processor is a spectrometer and the second optical processor comprises
a reflector, for deflecting reflected light from the wafer,
a series of waveguides arranged to intercept the deflected light, in such a way that different angles of reflection of the light from the wafer are indicated by different waveguides respectively giving maximum light intensity,
light intensity detectors associated with each waveguide to detect light intensity at each waveguide, thereby to determine which waveguide has a maximum intensity, and therefrom to derive the angle of reflection from the wafer.
The apparatus preferably comprises a stress calculator operatively associated with the second optical processor for calculating an indication of stress in the wafer, the indication being based on the bow.
Preferably, the light directors comprise optical heads operable to receive the reflections from the wafer and to direct the reflections to the second optical switch.
Preferably, the interferometer is a Michelson interferometer.
Preferably, the optical beam reflector is a parabolic reflector.
Preferably, the monochromatic light source comprises a laser.
The apparatus preferably comprises a stress determination unit operable to determine a stress level from the bow using the relationship       stress    =                  ED        2                    6        ⁢                  (                      1            -            v                    )                ⁢        tR              ,
wherein
E=Young""s modulus of the wafer,
v=Poisson""s ratio for the wafer,
Ds=thickness of the wafer,
R=net radius of curvature, derivable from the bow, and
Df=thickness of a film deposited on the wafer.
The apparatus preferably comprises a Fourier transform device connected to the spectrometer for transforming an output of the spectrometer into the frequency domain, thereby to obtain information of layer thickness of at least one deposited layer on the wafer.
According to a third aspect of the present invention there is provided a method of measuring stress in a silicon wafer, using an integrated measurement apparatus, the method comprising:
switching in a monochromatic light source to produce at least two monochromatic light beams,
reflecting the at least two beams of light from a surface of a wafer, a first beam from a central region of the wafer and a second beam from a peripheral region of the wafer,
determining properties of the reflected beams,
comparing the determined properties of the reflected light beams to thereby determine an extent of bowing in the wafer, and
calculating a stress level in the wafer from the extent of bowing.
Preferably, the properties are path lengths of the reflected beams.
Preferably, the determination and comparison comprise:
setting up an interference pattern between the reflections, and
analyzing the interference pattern.
Preferably, the determination and comparison comprise measuring and comparing respective angles of reflection of the beams.
Preferably, the method further comprises determining the stress level from the extent of bowing using the relationship       stress    =                  ED        2                    6        ⁢                  (                      1            -            v                    )                ⁢        tR              ,
wherein
E=Young""s modulus of the wafer,
v=Poisson""s ratio for the wafer,
Ds=thickness of the wafer,
R=net radius of curvature, derivable from the bow, and
t=thickness of a film deposited on the wafer.
Preferably, the light beams are laser light beams.
Preferably, the monochromatic light source is a laser light source.
Preferably, the method further comprises carrying out the determination and comparison using a Michelson interferometer.
According to a fourth aspect of the present invention there is provided a method of integrated measurement of stress and thickness in a semiconductor wafer, the method comprising:
selecting between a white light source and a monochromatic light source,
with the white light source:
irradiating a semiconductor at a single point,
receiving reflected light from the single point,
spectrally analyzing the reflected light using a transform,
deriving layer thickness from the analysis,
with the monochromatic source:
reflecting at least two beams of light from a surface of a wafer, a first beam from a central region of the wafer and a second beam from a peripheral region of the wafer,
determining properties of the reflected beams,
comparing the determined properties of the reflected light beams to thereby determine an extent of bowing in the wafer, and
calculating a stress level in the wafer from the extent of bowing.
Preferably, the properties are path lengths of the reflected beams.
Preferably, the determination and comparison comprise:
setting up an interference pattern between the reflections, and
analyzing the interference pattern.
Preferably, the determination and comparison comprise measuring and comparing respective angles of reflection of the beams.
Preferably, the method further comprises determining the stress level from the extent of bowing using the relationship       stress    =                  ED        2                    6        ⁢                  (                      1            -            v                    )                ⁢        tR              ,
wherein
E=Young""s modulus of the wafer,
v=Poisson""s ratio for the wafer,
Ds=thickness of the wafer,
R=net radius of curvature, derivable from the bow, and
t=thickness of a film deposited on the wafer.
Preferably, the method further comprises obtaining the monochromatic light beams from a laser light source.
Preferably, the method further comprises carrying out the determination and comparison using a Michelson interferometer.
Preferably, the method further comprises optically switching between the white and the monochromatic light sources.
Preferably, the method further comprises optically switching between a spectrometer and an interferometer.
Preferably, the method further comprises controlling the optical switching so that the spectrometer is selected with the white light source and the interferometer is selected with the monochromatic light source.
Preferably, the method further comprises switching light from either light source to an integrated beam former arrangement for irradiating the wafer.
Preferably, the integrated beam forming arrangement comprises a plurality of optical heads also usable to receive reflections from the wafer.